There are a few reasons.
We use whole herbs, and if we try to pack in too many herbs into one pill, the quantity of each herb will be very less. This will not be very effective - some minimum amount of each herb needs to be present. Hence, a reasonable combination (for a 500mg pill) cannot have more than 7-8 whole herbs in a single pill.
Having said that, having a smaller number of herbs in your supplement, does not mean that you are missing out on functional coverage. Herbs are multi-functional and a small set will still pretty much cover and support all your core functions. What is really important is that you have the right set of herbs (for you) in your combination, herbs that are best suited to your health and metabolic profile.